Achievement

Experimental realization of nanonetworks is challenging, and the most efficient and viable route appears to be by self-assembly. Although a number of chemical elements seem to play a key role in the formation of network nodes, little has been known about their role in microscopic processes governing branch formation. In this research, the role of sulfur during nanotube network growth was systematically studied, using a combination of theoretical and experimental techniques. Small concentrations of sulfur were shown to be sufficient to promote the formation of carbon heptagons during nanotube growth, and eventually to trigger the appearance of branching in nanotubes with stacked-cone morphologies. The network samples were characterized using high-resolution electron microscopy and detailed elemental analyses. The presence of minute amounts of sulfur was observed at the branching point between straight nanotube sections. These findings were rationalized and explained using molecular dynamics and other simulations based on density functional theory.

Significance

The
controlled assembly of elongated nanostructures into ordered
micronetworks constitutes a key challenge in nanotechnology.
These types of carbon structures are of tremendous interest
due to their fascinating mechanical and electronic properties
that could be used in the fabrication of nanodevices. This
research demonstrated that sulfur plays an important role in
the formation of branched nanotube networks with stacked-cone
morphologies. The growing branches possess minute amounts of
sulfur that are sufficient to promote the formation of heptagons
(negative curvature) and pentagons (positive curvature). This
approach can be expanded to include other chemical species
to further elucidate growth of carbon nanostructures.

The theory/modeling for this research was conducted at the Center for Nanophase Materials Sciences, which is sponsored by the Division of Scientific User Facilities, U.S. Department of Energy. This work is part of Jose Romo-Herrera’s PhD thesis, completed in part under the supervision of Vincent Meunier at CNMS (ORNL).

Figure 1: Theoretical calculations explaining the role of sulfur
at the atomic level.
a) Snapshots from quantum molecular dynamics simulations, showing
the effect of widening the CNT diameter in the presence of sulfur.
b) Stacked-cone structure promoted by sulfur widening the CNT.
The angle caused by the heptagons ((a), at 120 ps) is very close
to those observed in the stacked-cone morphologies (b). c) The
position of a sulfur atom within a carbon nanotube lattice marked
to indicate if it is in a pentagon (red), hexagon (blue), or heptagon
(green), and d) the total energy for the resulting system.

Figure
2: a) Low-magnification TEM pictures showing that every time
a branch opening occurs, there is a direction change
of the main stem (indicated by arrows and white dots). b) The final
Y-junction, along with high-resolution views of the structure at
the developed arms (panel B), and right at the angle between them
(panel A). c) Structure of the developing branch at the graphene
layer level. Inset: Fast Fourier transform calculation from the
image, allowing the angle between the stacking direction of the
graphene layers to be measured (34.5 °: stacking change marked
by left dashed line on HRTEM image, 37 °: stacking change at
the right dashed line).

Going
out on a limb: A combination of theoretical
techniques, high-resolution microscopy, and energy-dispersive X-ray
spectroscopy shows the role sulfur plays in branching phenomena
during carbon nanotube (CNT) network growth. A model is proposed
in which small amounts of sulfur are enough to trigger the growth
of a bud in a CNT, leading to kink formation and subsequent branch
growth.

**Section
Cover from Angewandte Chemie along with highlight provided by
the journal.